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Blog post for changed block tracking feature
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‎content/en/blog/_posts/2025-xx-xx-csi-changed-block-tracking.md‎

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---
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layout: blog
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title: 'Announcing Changed Block Tracking API support (alpha)'
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date: 2025-05-14T18:30:00+05:30
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draft: true
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slug: csi-changed-block-tracking
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author: >
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[Prasad Ghangal](https://github.com/PrasadG193) (Veeam Kasten)
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[Carl Braganza](https://github.com/carlbraganza) (Veeam Kasten)
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---
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We're excited to announce the alpha support for a _changed block tracking_ mechanism. This enhances
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the Kubernetes storage ecosystem by providing an efficient way for
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[CSI](https://kubernetes.io/docs/concepts/storage/volumes/#csi) storage drivers to identify changed
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blocks in PersistentVolume snapshots. With a driver that can use the feature, you could benefit
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from faster and more resource-efficient backup operations.
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If you're eager to try this feature, you can [skip to the Getting Started section](#getting-started).
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## What is changed block tracking?
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Changed block tracking enables storage systems to identify and track modifications at the block level
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between snapshots, eliminating the need to scan entire volumes during backup operations. The
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improvement is a change to the Container Storage Interface (CSI), and also to the storage support
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in Kubernetes itself.
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With the alpha feature enabled, your cluster can:
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- Identify allocated blocks within a CSI volume snapshot
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- Determine changed blocks between two snapshots of the same volume
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- Streamline backup operations by focusing only on changed data blocks
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For Kubernetes users managing large datasets, this API enables significantly more efficient
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backup processes. Backup applications can now focus only on the blocks that have changed,
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rather than processing entire volumes.
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{{< note >}}
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As of now, the Changed Block Tracking API is supported only for block volumes and not for
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file volumes. CSI drivers that manage file-based storage systems will not be able to
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implement this capability.
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{{< /note >}}
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## Benefits of changed block tracking support in Kubernetes
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As Kubernetes adoption grows for stateful workloads managing critical data, the need for efficient
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backup solutions becomes increasingly important. Traditional full backup approaches face challenges with:
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- _Long backup windows_: Full volume backups can take hours for large datasets, making it difficult
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to complete within maintenance windows.
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- _High resource utilization_: Backup operations consume substantial network bandwidth and I/O
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resources, especially for large data volumes and data-intensive applications.
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- _Increased storage costs_: Repetitive full backups store redundant data, causing storage
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requirements to grow linearly even when only a small percentage of data actually changes between
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backups.
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The Changed Block Tracking API addresses these challenges by providing native Kubernetes support for
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incremental backup capabilities through the CSI interface.
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## Key components
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The implementation consists of three primary components:
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1. _CSI SnapshotMetadata Service API_: An API, offered by gRPC, that provides volume
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snapshot and changed block data.
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2. _SnapshotMetadataService API_: A Kubernetes CustomResourceDefinition (CRD) that
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advertises CSI driver metadata service availability and connection details to
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cluster clients.
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3. _External Snapshot Metadata Sidecar_: An intermediary component that connects CSI
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drivers to backup applications via a standardized gRPC interface.
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## Implementation requirements
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### Storage provider responsibilities
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If you're an author of a storage integration with Kubernetes and want to support the changed block tracking feature, you must implement specific requirements:
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1. _Implement CSI RPCs_: Storage providers need to implement the `SnapshotMetadata` service as defined in the [CSI specifications protobuf](https://github.com/container-storage-interface/spec/blob/master/csi.proto). This service requires server-side streaming implementations for the following RPCs:
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- `GetMetadataAllocated`: For identifying allocated blocks in a snapshot
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- `GetMetadataDelta`: For determining changed blocks between two snapshots
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2. _Storage backend capabilities_: Ensure the storage backend has the capability to track and report block-level changes.
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3. _Deploy external components_: Integrate with the `external-snapshot-metadata` sidecar to expose the snapshot metadata service.
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4. _Register custom resource_: Register the `SnapshotMetadataService` resource using a CustomResourceDefinition and create a `SnapshotMetadataService` custom resource that advertises the availability of the metadata service and provides connection details.
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5. _Support error handling_: Implement proper error handling for these RPCs according to the CSI specification requirements.
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### Backup solution responsibilities
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A backup solution looking to leverage this feature must:
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1. _Set up authentication_: The backup application must provide a Kubernetes ServiceAccount token when using the
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[Kubernetes SnapshotMetadataService API](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/3314-csi-changed-block-tracking#the-kubernetes-snapshotmetadata-service-api).
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Appropriate access grants, such as RBAC RoleBindings, must be established to authorize the backup application
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ServiceAccount to obtain such tokens.
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2. _Implement streaming client-side code_: Develop clients that implement the streaming gRPC APIs defined in the
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[schema.proto](https://github.com/kubernetes-csi/external-snapshot-metadata/blob/main/proto/schema.proto) file.
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Specifically:
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- Implement streaming client code for `GetMetadataAllocated` and `GetMetadataDelta` methods
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- Handle server-side streaming responses efficiently as the metadata comes in chunks
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- Process the `SnapshotMetadataResponse` message format with proper error handling
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The `external-snapshot-metadata` GitHub repository provides a convenient
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[iterator](https://github.com/kubernetes-csi/external-snapshot-metadata/tree/master/pkg/iterator)
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support package to simplify client implementation.
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3. _Handle large dataset streaming_: Design clients to efficiently handle large streams of block metadata that
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could be returned for volumes with significant changes.
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4. _Optimize backup processes_: Modify backup workflows to use the changed block metadata to identify and only
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transfer changed blocks to make backups more efficient, reducing both backup duration and resource consumption.
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## Getting started
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To use changed block tracking in your cluster:
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1. Ensure your CSI driver supports volume snapshots and implements the snapshot metadata capabilities with the required `external-snapshot-metadata` sidecar
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2. Make sure the SnapshotMetadataService custom resource is registered using CRD
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3. Verify the presence of a SnapshotMetadataService custom resource for your CSI driver
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4. Create clients that can access the API using appropriate authentication (via Kubernetes ServiceAccount tokens)
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The API provides two main functions:
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- `GetMetadataAllocated`: Lists blocks allocated in a single snapshot
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- `GetMetadataDelta`: Lists blocks changed between two snapshots
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## What’s next?
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Depending on feedback and adoption, the Kubernetes developers hope to push the CSI Snapshot Metadata implementation to Beta in the future releases.
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## Where can I learn more?
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For those interested in trying out this new feature:
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- Official Kubernetes CSI Developer [Documentation](https://kubernetes-csi.github.io/docs/external-snapshot-metadata.html)
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- The [enhancement proposal](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/3314-csi-changed-block-tracking) for the snapshot metadata feature.
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- [GitHub repository](https://github.com/kubernetes-csi/external-snapshot-metadata) for implementation and release status of `external-snapshot-metadata`
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- Complete gRPC protocol definitions for snapshot metadata API: [schema.proto](https://github.com/kubernetes-csi/external-snapshot-metadata/blob/main/proto/schema.proto)
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- Example snapshot metadata client implementation: [snapshot-metadata-lister](https://github.com/kubernetes-csi/external-snapshot-metadata/tree/main/examples/snapshot-metadata-lister)
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- End-to-end example with csi-hostpath-driver: [example documentation](https://github.com/kubernetes-csi/csi-driver-host-path/blob/master/docs/example-ephemeral.md)
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## How do I get involved?
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This project, like all of Kubernetes, is the result of hard work by many contributors from diverse backgrounds working together.
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On behalf of SIG Storage, I would like to offer a huge thank you to the contributors who helped review the design and implementation of the project, including but not limited to the following:
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- Ben Swartzlander ([bswartz](https://github.com/bswartz))
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- Carl Braganza ([carlbraganza](https://github.com/carlbraganza))
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- Daniil Fedotov ([hairyhum](https://github.com/hairyhum))
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- Ivan Sim ([ihcsim](https://github.com/ihcsim))
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- Nikhil Ladha ([Nikhil-Ladha](https://github.com/Nikhil-Ladha))
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- Prasad Ghangal ([PrasadG193](https://github.com/PrasadG193))
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- Praveen M ([iPraveenParihar](https://github.com/iPraveenParihar))
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- Rakshith R ([Rakshith-R](https://github.com/Rakshith-R))
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- Xing Yang ([xing-yang](https://github.com/xing-yang))
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Thank also to everyone who has contributed to the project, including others who helped review the
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[KEP](https://github.com/kubernetes/enhancements/pull/4082) and the
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[CSI spec PR](https://github.com/container-storage-interface/spec/pull/551)
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For those interested in getting involved with the design and development of CSI or any part of the Kubernetes Storage system,
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join the [Kubernetes Storage Special Interest Group](https://github.com/kubernetes/community/tree/master/sig-storage) (SIG).
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We always welcome new contributors.
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The SIG also holds regular [Data Protection Working Group meetings](https://docs.google.com/document/d/15tLCV3csvjHbKb16DVk-mfUmFry_Rlwo-2uG6KNGsfw/edit).
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New attendees are welcome to join our discussions.

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